FOLLICULAR DETERMINANTS OF THE LENGTH AND DIAMETER OF WOOL FIBERS .1. COMPARISON OF SHEEP DIFFERING IN FIBER LENGTH/DIAMETER RATIO AT 2 LEVELS OF NUTRITION

Two groups of sheep were formed with similar mean fibre diameter (D) but differing rate of fibre elongation (L), and two groups with similar mean L but differing D. The sheep were placed on a low plane of nutrition for 8 weeks followed by a high plane of nutrition for a further 7 weeks. L, as a result of selection and nutrition, ranged from 268 to 515 mu m/day, D from 17.5 to 32.8 mu m, and L/D ratios from 13.1 to 24.5 between individuals. Nutrition significantly influenced the dimensions of the follicle bulb and dermal papilla, the rate of division of follicle bulb cells, the dimensions of the cortical cells, and the production ratio (the ratio of fibre area to fibre-plus-inner root sheath area). The high and low diameter groups differed in bulb and papilla dimensions and in the rate of bulb cell division. The high and low fibre length growth phenotypes differed only in papilla length and cortical cell length. Stepwise multiple regression analysis revealed that a model including terms for nutrition, phenotype group and cortical cell length accounted for 0.60 of the variance in fibre L (P < 0.0001). Fibre D was best accounted for by nutrition, phenotype group, cortical cell volume and papilla area (R(a)(2) = 0.88, P < 0.0001). This differential dependence of the fibre dimensions on follicle characters means that the shape of the fibre has the potential to change in situations where cortical cell length, cortical cell volume and papilla size are differentially altered. Several hormonal, nutritional and selection regimes known to alter L/D must have this effect, but simple nutritional changes such as the one imposed in this experiment apparently do not, as there was no change in fibre L/D. The follicular efficiency of fibre production (measured as fibre output per unit bulb tissue or per unit bulb cell produced) in sheep producing fine (20.3 mu m) wool was similar to that of sheep producing coarse (27.0 mu m) wool. There appears therefore to be no inherent inefficiency in small follicles producing fine wool and thus no follicular impediment to the production of large amounts of fine wool.